Midlife risk factors have been shown to have effects on both physical functional status and cognitive functioning in old age, although the mechanisms whereby these factors have their impact remain to be elucidated. The NHLBI Twins Study offers a unique opportunity by which to analyze factors that influence change in physical performance in old age. The cohort consists of monozygotic and dizygotic twins recruited between 1969 and 1972 and examined periodically thereafter. In the fourth follow-up, done between 1995 and 1997, the EPESE physical performance battery was added to the protocol. This battery assesses lower extremity functional limitations through simple tests of gait, ability to rise from a chair, and balance. Initial analyses of Twin Study data revealed substantial heritability of physical performance. The fifth follow-up, done in 1999 and 2000, also included the performance battery and there is now opportunity to examine change in physical performance. Methodologic work has shown that the battery is a reliable and sensitive measure of change in functioning. Subjects were average age 73 when they had their performance first measured in 1995-97. Current work on the study will examine a number of questions relevant to physical functioning in old age: (1) How are midlife cardiovascular risk factors related to change in physical performance at old age? (2) If these midlife risk factors, particularly high blood pressure, do influence old age functioning, is this explained by changes in brain white matter lesions and stroke identified at old age on magnetic resonance imaging (MRI)? (3) What is the heritability of change in physical performance?